1. Field of the Invention
The present invention relates to the conversion of chemical energy to electrical energy and, more particularly, to a cathode of silver vanadium oxide (SVO) admixed with a combination of polymeric binders. Importantly, the silver vanadium oxide admixture is coated to a current collector for incorporation into an electrochemical cell. Prior to the present invention, SVO could only be contacted to a current collector as a pressed powder or as a free-standing sheet. The preferred binder formulation is a mixture of a halogenated polymeric binder and a polymide binder, most preferably polyvinylidene fluoride (PVDF) and a polyimide. The present electrodes built from SVO containing the mixed polymeric binders are useful in both primary and secondary cells discharged at elevated and low temperatures.
2. Prior Art
Halogenated polymeric materials have been used extensively as binders for electrodes in all types of nonaqueous electrochemical cells, and particularly lithium cells. For example, polyvinylidene fluoride is a material that functions well as an electrode active binder at or near ambient temperature. However, PVDF is soluble in organic electrolytes at elevated temperatures. Thus, cells manufactured with PVDF as the sole binder material cannot be used in high temperature applications or survive high temperature exposure, such as occurs in an autoclave, without severe degradation.
It is also known in the prior art to employ non-halogenated polymeric materials as binders in nonaqueous, alkali metal electrochemical cells. Exemplary is U.S. Pat. No. 5,468,571 to Asami et al. which discloses that polyimide (PI) is useful as an anode binder in lithium secondary cells. Electrodes containing such non-halogenated polymers as the sole binder material are somewhat brittle and have a tendency to crack.
U.S. Pat. No. 6,001,507 to Ono et al. describes electrodes for secondary cells prepared from a mixture of a soluble polyimide and PVDF combined with an active material such as LiCoO2. The soluble polyimide is a material which is converted to the imide before it is mixed with the depolarizing active mixture. This is done to prevent water from entering the electrochemical system. However, fabricating an electrode with binders which are soluble in nonaqueous solvents hinders the active mixture/current collector contact interface, especially after repeated cycling.
Thus, there is a need for a binder formulation that is insoluble in both primary and secondary organic solvent electrolyte systems, particularly those used to activate alkali metal primary cells or alkali metal ion secondary cells, and is able to withstand high temperature exposure without compromising discharge efficiency.
U.S. Pat. No. 5,571,640 to Takeuchi et al. describes a process for manufacturing a cathode component from a free-standing sheet of active material, such as of SVO. The comminuted SVO is combined with a binder and conductive additives suspended in a solvent to form a paste. The paste is then fed into a series of roll mills to form a silver vanadium oxide sheet, which is then pressed to a current collector. This patent is described as being an improvement over building cathodes from a powdered silver vanadium oxide pressed to a current collector.
Prior to the present invention, it was not possible to build a silver vanadium oxide cathode by other than pressing a powder or a free-standing sheet of the active material to a current collector. As will be described in detail hereinafter, coating a slurry of electrode active material, particularly of silver vanadium oxide, to a current collector results in improved discharge efficiency, especially during pulsing, in comparison to the prior art techniques. This improvement is believed to be directly attributable to the novel binder mixture.
The present invention is, therefore, directed to an electrode built of an active material including a mixture of two polymeric binders useful in nonaqueous organic electrolytes activating alkali metal or alkali metal ion electrochemical cells, and a method that provides flexible, no-brittle electrodes, particulary of silver vanadium oxide, dischargeable at elevated tempratures. The first binder is preferably a halongenated polymeric binder and, more preferably, a fluorinated polymeric material, such as PVDF. The second binder is polyimide, and preferably one derived from heating a polyamic acid prior to activating the electrochemical couple. A pereferred binder mixture is PVDF and PI.
To construct an electrode, for example of a mixture of silver vanadium oxide, polyamic acid and PVDF, and possibly a conductive diluent, the active admixture is first formed into a slurry having the consistency of a milk shake and coated to a current collector. This electrode assembly is then heated to convert the polyamic acid to an insoluble polyimide prior to being incorporated into an electrochemical cell.
These and other aspects of the present invention will become increasingly more apparent to those of ordinary skill in the art by reference to the following description taken in conjunction with the accompanying drawings.